Montmorillonite in Heping County, Guangdong Province, was studied by chemical analysis, DAT, TG, XRD, IR and EPR. It is found that montmorillonite dehydrates adsorbed water and interlayer water when the temperature is between 126~148℃, and that this process is reversible. When the thermal treatment temperature reaches 659℃, the hydroxyl in octahedra sheet begins dehydrating, but the layer structure remains unchanged. When the temperature reaches 900℃, the layer structure of montmorillonite is destroyed, and the new mineral phase SiO2_χis formed; when the temperature reaches 1200℃, cristobalite and mullite appear;when the temperature reaches 1350℃, cordierite occurs. There are two main signals in montmorillonite EPR: one (g=4.16) is a sharp peak corresponding to the structural Fe3+ions of octahedron sheet in montmorillonite, and the other (g=2.1) is a broad peak corresponding to Fe3+of the amorphous oxide and hydroxide adsorbed on montmorillonite surface. Both peaks change greatly after thermal treatment at different temperatures. When the temperature is lower than 500℃, the signal with g=4.16 does not show obvious change; when the temperature is higher than 900℃, the change becomes remarkable; when the temperature equals 1350℃,the signal decreases again and a new signal with g=9.39 appears. The signal with g=2.1 weakens gradually and eventually disappears with the rising temperature. This change is attributed to the structural change and migration of Fe3+ions in montmorillonite.